1. Field of the Invention
The invention relates to a low-temperature sintered barium titanate microwave dielectric ceramic materials, and in particular, to a low-temperature sintered barium titanate microwave dielectric ceramic materials manufactured by adding a low melting glass into Ba2Ti9O20 as a host material so as to reduce the sintering temperature.
2. Description of the Prior Art
Conventionally, ceramic materials are co-fired with a low melting electrode materials to form a module. However, since the ceramic material has a very high sintering temperature (for example greater than 1300° C.), the application of using the electrode material with a low melting point and high electrical conductivity, e.g. silver, copper or gold, is restricted. Therefore, for a purpose of developing a new-generation high frequency microwave module, the current trend is to reduce the sintering temperature of the ceramic material.
For achieving the principal object of reducing the sintering temperature of the ceramics , three methods are provided. The first method is a liquid-phase sintering by adding a low melting glass. The second method is to synthesize a very fine powder by using chemical processes, which increases the overall surface area and thus reduces the sintering temperature. The third method is to develop a new material capable of being sintered at a low temperature. Although the diameter and the composition of the powders can be precisely controlled by the chemical method, such chemical method is not suitable for mass production and thus the manufacturing cost there is high. In addition, it takes much time to develop a new material. Therefore, adding a low melting glass is widely used. In a case that the material Ba2Ti9O20 is sintered at a low temperature (for example less than 1,000° C.), once such material is applied to a low-temperature co-fired ceramic (LTCC) or multilayer ceramic (MLC) substrate, it can be co-fired with the electrode material having a low melting point and high electrical conductivity. By using an Ag-Pd electrode with a high Ag content, the cost and volume of the module element are reduced, and the performance thereof is increased.
As shown in the phase diagram of FIG. 1, the range of a pure Ba2Ti9O20 is very narrow, and it is very difficult to obtain the pure Ba2Ti9O20 phase when the conventional method of mixing solid powders is used. In Journal of American Ceramic Society, volume 74 (8), pages 1841-1847, 1991, Kirby et al. describes compounds with high TiO2 contents in a BaO-TiO2 system, for example BaTi5O11, Ba4Ti3O30, BaTi4O9, Ba2Ti9O20, etc. Among these compounds, Ba2Ti9O20 the most excellent microwave dielectric properties, and is suitable to be used as a dielectric oscillator. The single-phase Ba2Ti9O20, however, is difficult to be synthesized because other barium titanate compounds are co-exist.
U.S. Pat. No. 3,938,064 describes a method for preparing a low-loss dielectric material Ba2Ti9O20. The starting powder is 18˜19mole % BaCO3 powder, which is added to a test specimen made of a TiO2 powder under an oxygen-containing atmosphere (i.e. the oxygen content is greater than 90%) and calcined at a temperature ranged between 900 and 1400° C. for a period of from 10 to 100 hrs. Alternatively, the mixed powders can be calcined at a temperature ranged between 1300 and 1420° C. under an oxygen-containing atmosphere. The feature of this patent is that calcining and sintering processes are carried out at a selected barium/titanium ratio and under an oxygen-containing atmosphere. During a solid reaction, some oxide additives are added to promote synthesis of Ba2Ti9O20 phase, and thus a single-phase Ba2Ti9O20 is obtained.
In Journal of American Ceramic Society, volume 41 (10), pages 390-394, 1958, Jonker describes the addition of SnO2 and ZrO2 as phase stabilizers and a calcination reaction carried at a temperature of 1300° C. so as to synthesize a single-phased Ba2Ti9O20. In Japanese Journal of Applied Physics, Volume 22 (7), pages 1125-1128, Shoichiro describes the addition of Mn into Ba2Ti9O20, and the mixture is sintered at a high temperature of 1400° C. for 3 hrs so as to obtain most Ba2Ti9O20. In Journal of Materials Science Letters, Volume 11, pages 457-459, 1992, Chandar describes the addition of excess TiO2 to stabilize the Ba2Ti9O20 phase, however, which is obtained at a sinte ring temperature of 1200° C. for 4 hrs. In Journal of American Ceramic Society, Volume 77 (4), pages 1052-1056, Yu describes the addition of oxides such as SnO2, A1203, Bi2O3 to the stabilize formation of the Ba2Ti9O20 phase. However, the single-phased Ba2Ti9O20 is obtained at a sintering temperature of 1300° C. for 6 hrs.
U.S. Pat. No. 4,353,047 describes an dielectric material having (1−x)BaO. TiO2 as a principal component, wherein 0.7≦×≦0.95. To the dielectric material, 0.05˜0.3wt % of MnCO3 and 1.0˜5.0wt % of ZrO2 are added, mixed and sintered at 1250˜1420° C. in the air or an oxygen atmosphere. Then, by using the manganese oxide and zirconium oxide, improved dielectric properties, for example a dielectric constant of 39˜89, τf value of +3˜+130 ppm/° C. and Q value of 3700˜6600, are obtained.
U.S. Pat. No. 6,642,167 describes an insulating ceramic material, the principal component thereof being represented by the formula BaO-x{(1−y)TiO2.yZrO2}, where 3.5≦x≦4.5, 0≦y≦2, and the auxiliary component thereof comprising 5˜30wt % of ZnO, 0.1˜3wt % of R2O (R is an alkali metal), 0.5˜6wt % of SiO2, 0.1˜7wt % of CuO, 0.1˜6wt % of V2O5 or Bi2O3. This ceramic material can be simultaneously sintered together with silver or copper electrodes at a temperature of 1,000° C. or less, and exhibits a high dielectric constant, a high Q value, a small change in capacitance value with temperature, and high environmental resistance. In the Japan Society of Applied Physics, 36, pages L1604-07, 1997, Cheng et al. describe the use of a 5MgO-19CaO-26Al2O3-50SiO2 (MCAS) glass as a sintering aid. In this study, it is found that the addition of the MCAS glass resulted in enhanced liquid-phase sintering and reduced the sintering temperature of the Ba2Ti9O20 ceramic. However, the sintering temperature is also greater than 1100° C. In addition, after the addition of the MCAS glass, the main phase of the sintered ceramic material is still Ba2Ti9O20 phase. The εr is increased as temperature is increased, but decreased as the addition of the MCAS glass is decreased. In Materials Research Bulletin, 35(14-15), pages 2445-56, 2,000, Huang et al. describe the use of the simple glasses B2O3 and SiO2 and the commercial glass PBS (PbO-B2O3-SiO2) as liquid-phase sintering aids so as to effectively reduce the sintering temperature. After B2O3 and SiO2 are added and the mixture is sintered at 1200° C., 95% of theoretical density is obtained. Whereas, after PBS is added and the mixture is sintered at 1200° C., a single-phased Ba2Ti9O20 is obtained. With regards to the dielectric properties, after the mixture is sintered at 1200° C., the addition of 5wt % B2O3 and PBS offer εr=36.5 and 37.2, Q=6700 and 1650 (at 6 GHz ), τf=38 and 9 ppm/° C., respectively. However, since this type of glass contains PbO, the application thereof is limited due to the requirement of environmental protection. In Journal of European Ceramic Society, pages 2259-2563, 2003, Huang et al. describe a single phased Ba2Ti9O20 is obtained after 2 mol % of SnO2 is added and the mixture is sintered at 1350° C. When 50 vol % of a BaBSiO (42BaO-45B2O3-13SiO2) glass is added, and then holding at the temperatures of 800° C., 850° C. and 900° C., respectively, for 1 hr, the second phase BaTi(BO3)2 is produced, which has no influence on the microwave properties of Ba2Ti9O20. After the mixture is sintered at 900° C./30 min, the following microwave properties are obtained: εr=3.2, Q×f=1150.
As known from the above literatures, in order to apply the production processes of the low-temperature co-fired ceramic (LTCC) and multilayer ceramic (MLC) substrate, it is necessary to reduce the sintering temperature of the Ba2Ti9O20 ceramic material as well as maintain good microwave properties. However, the above technologies fail to meet the requirement due to the following problems:
1. The purposes of reducing the sintering temperature of the Ba2Ti9O20 ceramic material and maintaining good microwave properties are not simultaneously attained. If good microwave properties are maintained, the sintering temperature fails to be effectively reduced. Likewise, if the sintering temperature is reduced, the dielectric constant (εr) fails to be maintained, and the good microwave properties, for example τf≦±30 ppm/° C. and Q×f>5,000, are not achieved.
2. Since the sintering temperature is still high (>1,000° C.), the application of being co-fired with the metallic electrode material having a low melting point and a low cost is not acceptable.
3. The addition of heavy metal which is necessary according to the technologies disclosed in the literatures, does not meet the requirement of environmental protection.